The integrated circuit technology has a need to obtain narrow line widths in the range of 1 micrometer or less by extending standard photolithography techniques and avoiding the need to use the more expensive and complex techniques such as electron beam or X-ray lithography. One such technique is described in H. B. Pogge in IBM Technical Disclosure Bulletin, November 1976, Vol. 19, No. 6, pp.2057-2058, entitled "Narrow Line Widths Masking Method". This method involves the use of a porous silicon followed by the oxidation of the porous silicon. Another technique is described by S. A. Abbas et al. in the IBM Technical Disclosure Bulletin, Vol. 20, No. 4, September 1977, pp. 1376-1378. This method describes the use of polycrystalline silicon masking layers which are made into mask by first using an intermediate mask of oxidation blocking material, such as silicon nitride in the formation of polycrystalline silicon. Line dimensions below about 2 micrometers may be obtained by this technique.
U.S. Pat. Nos. 4,209,349 and 4,209,350 by I. T. Ho et al., U.S. Pat. No. 4,234,362 by J. Riseman and U.S. Pat No. 4,256,514 by H. B. Pogge describe methods for forming narrow dimensioned, for example, sub-micrometer regions on a silicon body. These patents all involve the formation of substantially horizontal surfaces and substantially vertical surfaces on a silicon body and then forming a layer of a very narrow dimension on both the substantially horizontal and substantially vertical surfaces. This layer is then subjected to an anisotropic etching process such as by reactive ion etching, to substantially remove the horizontal layer while leaving the vertical layer substantially intact. The vertical layer dimension is adjusted depending upon the original thickness of the layer applied. In this way such a narrow dimension region as 1 micrometer or less is obtained.
There has been significant effort in the integrated circuit field to develop processes for making sub-micrometer channel length field effect transistor with a high degree of channel length control. Examples of this work are described in "A New Edge-defined Approach for Sub-micrometer MOSFET Fabrication" by W. R. Hunter et al., IEEE Electron Device Letters, Vol. EDL-2 No. 1, January 1981, pp. 4-6, "Sub-micrometer Polysilicon Gate CMOS/SOS Technology" by A. C. Ipri et al. published in IEEE Transactions on Electron Devices, Vol. ED-27, No. 7, July 1980, pp. 1275-1279 and "A Novel Sub-micron Fabrication Technique" by T. N. Jackson et al. published in IEDM 1979 Conference Volume, pp. 58-61. The first paper relies on the reactive ion etching technique to form a sidewall silicon dioxide. The second paper utilizes a technique involving lateral diffusion of boron. The third method uses the plating of a metal on the edge of a conventionally patterned metal layer. Other short channel field effect transistor devices are illustrated in the W. E. Armstrong U.S. Pat. No. 4,062,699; J. Goel U.S. Pat. No. 4,145,459 and J. H. Scott, Jr. U.S. Pat. No. 4,201,603. The Armstrong patent utilizes an ion implantation and diffusion process to narrow the channel length of his MOSFET. The Goel patent utilizes a process sequence that involves the use of a recess formed in the portion of the semiconductor body and further involves the plating of metal films on each side of the recess until the spacing between the metal films across the recess is equal to desired length of the gate. The Scott, Jr. patent controllably dopes an edge of a polysilicon layer and then is able to remove the undoped polysilicon by etching it with a material which does not etch the doped polysilicon region.
A particularly effective MOS FET configuration allowing densities and performance higher than that heretofore available in such devices is described in "A New Short Channel MOS FET with Lightly Doped Drain" by Saito et al. in Denshi Tsushin Rengo Taikia (Japanese), April 1978, p. 2-20. The LDD N channel MOS FET includes, in addition to the channel separating implanted N+ source and drain regions, the sub-micrometer diffused N- regions, which increases the channel breakdown voltage or snap-back voltage and reduces device drain junction electron impact ionization (and thus, hot electron emission) by spreading the high electric field at the drain pinch-off region into the N- region. This allows either an increase in power supply voltage or reduction in channel length at a given voltage to achieve performance enhancement. An improved process for making such a device is given in patent application Ser. No. 06/217,497 filed Dec. 17, 1980 by S. Ogura and P. J. Tsang and entitled "Method of Fabricating High Speed High Density MOS Dynamic RAM with Lightly Doped Drain" in which the N- LDD region of the device is formed by a controlled N- ion implantation and the forming of sub-micrometer wide SiO.sub.2 sidewall spacers abutting to the gate. Other lightly doped drain processes are given in the I. T. Ho and J. Riseman U.S. Pat. Nos. 4,209,349 and 4,209,350. These patents also show self-aligned diffused regions formed by outdiffusion from layers formed on the surface of a semiconductor substrate into the substrate. In the above mentioned Ogura and Tsang's patent application, the polycrystalline silicon gate plate of the LDDFET is formed by conventional lithographic process. Its minimum achievable length is limited by the capability of the lithographic tool used. In the present invention, on the other hand, the self-aligned sidewall formation technique is repeatedly used to form the sub-micrometer length gate and the LDD sidewall spacers of the device. The minimum achievable device gate length is no longer limited by the lithographic tools but can be set by design requirement. Devices with channel length less than 0.5 micrometers can be readily made with conventional photolithographic tool.
It is therefore desirable to provide a high density, short channel field effect transistor which can be integrated into integrated circuit structures that is useful in memory or logic. It is also useful to have such short channel field effect transistors isolated from one another with dielectric isolation.